I'm getting ready to try it on a case, probably next weekend. The case is evenly yellowed on the front, so hopefully the results will come out even.

I just realized: I could test this on the old Lambda/O2 sensor counter in my car under the speedometer cable. It's made of the same, basic plastic from the 80's and it's hideously yellow. Eeeexcellent! -Smithers voice-

Yes I have used it quite a lot on PC's and Amigas.

You need to keep a close eye on it as if you leave it in the sun too long it will over-oxidise the plastic leaving white splotches which can't be removed short of painting over them. In Australia t he sun is pretty harsh so a safer method may be to get a lamp with a UV blacklight bulb.

Rather than mix up the retrobright mixture, I've actually found this to be somewhat effective as well:
buy a 1KG oxy based detergent which active ingredient over 30% (I can get these at my supermarket for about $5), dump it in a tub, put in your computer parts and fill with just enough water to submerge them (and disolve the detergent).

Leave it for a week and it will actually take the edge off the yellowing with little risk of damaging the plastic. If its severely yellowed you may need to mix up the proper retrobright mixture. This is a good starting point though because it will clean any dirt and grime off before the retrobright treatment.

Hmm I'm interested in this topic too, I have a couple of cases and things that I refuse to use with that horrible yellow colour. In general I struggle to follow recipes for some reason so haven't tried the exact retr0bright formula / process. But I've recently tried an adhoc mix of peroxide bought at a hairdressers, and napisan (an oxy based detergent). It foams up satisfyingly, particularly in the sun, and has reduced the yellowing significantly. Not completely though, and given that I'm trying to match yellowed drive bay panels to a case which is otherwise a perfect beige, it's a little frustrating.

I've given up for now, but would be interested to hear other people's experiences.

^ I never had much luck actually matching colours. I don't think you'll ever completely get rid of the yellowing 100%. Indeed, retrobrite is only temporary, the yellow process will happen again over time.

Paint might be another option. I took one of my drive bay covers into a paint store and they could match it using their colour match machine. I used it to repaint a scratched up metal case cover but if you sand and prep the plastic properly it should work on that too, maybe use a dullcoat lacquer over the top to seal it.